class Solution(object):
    def findTheDifference(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: str
        """
        sDict = {}
        for c in s:
            if c not in sDict:
                sDict.update({c:0})
            sDict[c]+=1
        for c in t:
            if c not in sDict:
                return c
            sDict[c]-=1
            if sDict[c]==0:
                del sDict[c]
        